Currently a number of activities are ongoing to define requirements on the next generation mobile network (e.g. Fifth Generation (5G)). A key requirement for the next generation mobile networks is flexibility to support multiple use cases. The next generation mobile network may be based on e.g. Software Defined Networking (SDN) that enables a solution with a strict division between control plane and user plane. This allows for flexibility to rapidly deploy network service functions and to support multiple use cases. The architecture of the next generation mobile network may support traffic aggregation on a larger granularity than per-device tunnels. This allows for efficiency and scalability required to support the massive amount of devices in the 5G time frame.
The current Evolved Packet Core (EPC) architecture is optimized for the mobile broadband use case where traffic for an end user passes a Packet Data Network Gateway (PGW) acting as mobility anchor point. It is limited in its flexibility to support new use cases due to the time consuming standardization process. Furthermore, it is envisioned that the 5G core network architecture will have to handle many more devices, which may make today's EPC inefficient since it maintains at least one tunnel per device.
Roaming in 3GPP EPC Architecture
Roaming for the current Third Generation Partnership Project (3GPP) Evolved Packet Core (EPC) roaming architecture is defined by the 3GPP. FIG. 1 shows an example of a roaming architecture with its interfaces. The so-called home-routed roaming is where the UE 101 visits a visited network 100a (e.g. a Visited Public Land Mobile Network (VPLMN, V-PLMN)) and its traffic gets routed through a PGW in the home network 100b (e.g. Home Public Land Mobile Network (HPLMN, H-PLMN)) of the UE 101.
FIG. 1 shows the UE 101 which communicates via a Radio Access Network (RAN) to one or more Core Networks (CNs). The RAN is represented by an Evolved-Universal Terrestrial Radio Access Network (E-UTRAN) 105 in FIG. 1. The E-UTRAN 105 comprises elements such as a RAN node (not shown in FIG. 2). The RAN node may be for example a Base Station (BS), a NodeB, an evolved NodeB (eNode B, eNB), Radio Network Controller (RNC) or any other element capable to communicate with a UE 101. The reference point between the UE 101 and the E-UTRAN 105 may be referred to as LTE-Uu.
A Mobility Management Entity (MME) 108 located in the visited network 100a may be connected to the E-UTRAN 105 via the reference point S1-MME. The MME 108 is an element having function such as e.g. Non-Access Stratum (NAS) signalling, Inter CN node signalling for mobility between 3GPP access networks, UE reachability, Tracking Area (TA) list management, PGW and Serving GateWay (SGW) selection, MME selection for handover with MME change etc. S10 is the reference point between MMEs 108 for MME relocation and MME to MME information transfer.
Two gateways 110 are seen in FIG. 1, i.e. the SGW 110 and the PGW 113. The SGW 110 is located in the visited network 100a and the PGW 113 is located in the home network of the UE 101. The SGW 110 is the gateway which terminates the interface towards E-UTRAN 105. The reference point between the SGW 110 and the E-UTRAN 105 for the per bearer user plane tunneling and inter eNB path switching during handover may be referred to as S1-U. The SGW 110 routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNB handovers and as the anchor for mobility between LTE and other 3GPP technologies (relaying the traffic between Second Generation/Third Generation (2G/3G) systems and the PGW 113) etc. S11 is the reference point between the SGW 110 and the MME 108.
The PGW 113 is the gateway which terminates the SGi interface towards the Packet Data Network (PDN). The PDN is illustrated in FIG. 1 by the Operator's IP Services (e.g. IMS, PSS etc.) 114. IP is short for Internet Protocol, IMS is short for IP Multimedia Subsystem or IM Multimedia core network Subsystem and PSS is short for Packet Switched Streaming. If the UE 101 is accessing multiple PDNs, there may be more than one PGW 113 for that UE 101. Functions of the PGW 113 are e.g. providing connectivity from the UE 101 to external PDNs by being the point of exit and entry of traffic for the UE 101, performing policy enforcement, packet filtering for each user, charging support, lawful interception and packet screening etc. S5/S8 is the reference point which provides user plane tunneling and tunnel management between the SGW 210 and the PGW 125. S8 is used when the UE 101 roams between different operators while S5 is a network internal interface.
The SGSN 115 is located in the visited network 100a and is responsible for the delivery of data packets from and to the UE's 101 within its geographical service area. SGSN 115 is short for Serving GPRS Support Node, where GPRS is short for General Packet Radio Services. One of the SGSN's 115 functions is to provide signaling for mobility between 2G/3G and E-UTRAN 3GPP access networks. 2G/3G access network are exemplified with GSM EDGE Radio Access Network (GERAN) 118 and Universal Terrestrial Radio Access Network (UTRAN) 120 in FIG. 1. GSM is short for Global System for Mobile communications and EDGE is short for Enhanced Data rates for Global Evolution. Some further functions of the SGSN 115 are to handle packet routing and transfer, mobility management (attach/detach and location management), logical link management, and authentication and charging functions etc. S3 is the interface between the SGSN 115 and the MME 108. S4 is a reference point between the SGSN 115 and the SGW 110. S12 is the reference point between the SGW 110 and the UTRAN 120. In some embodiments, the SGSN 115 and the MME 108 are co-located in one node.
The Home Subscriber Server (HSS) 123 is located in the home network 100b and is a subscriber server node similar to the GSM Home Location Register (HLR) and Authentication Centre (AuC). The HSS 123 comprises subscriber-related information (subscriber profiles), performs authentication and authorization of the user, and may provide information about the subscriber's location and IP information. The reference point S6a enables transfer of subscription and authentication data for authenticating/authorizing user access to the evolved system between the MME 108 and the HSS 123. Note that the HSS 123 is only shown as an example in FIG. 1 and that any type of subscriber database may be used instead of the HSS 123, such as e.g. a. HLR etc.
The Policy and Charging Rules Function (PCRF) 125 is located in the home network 100b and is a policy and charging control element. The PCRF 25 encompasses policy control decision and flow based charging control functionalities, it provides network control regarding the service data flow detection, gating, Quality of Service (QoS) and flow based charging etc. The PCRF 125 may be described as a functional entity which may be a standalone node or a function implemented in another node. The reference point Gx provides transfer of (e.g. QoS) policy and charging rules from the PCRF 125 to e.g. a Policy and Charging Enforcement Function (PCEF) in the PGW 113. Rx is the reference point which resides between the PCRF 125 and the Operator's IP Services 114.
The key tasks that are solved in the roaming architecture illustrated in FIG. 1 include:                An IP Address needs to be attained/allocated for the UE 101, over the S8 interface.        To ensure that GPRS Tunneling Protocol (GTP) packets are sent to and received from the home network 100b, over the S8 interface.        To update the Subscriber Location in the HSS 123, over the S6a interface.        
FIGS. 2a and 2b are a signaling diagrams which describes an example of how an attach procedure is performed. FIG. 2a illustrates steps 201-222 and FIG. 2b illustrates steps 223-241, steps 201-222 is performed first and then steps 223-241 are performed. The UE 101 needs to register with the network to receive services that require registration. This registration is described as attachment. In the home-routed roaming case, some of the message exchanges will be over roaming interfaces. E.g. steps 213 and 214 will be over the S8 interface between the SGW 110 in the visited network 100a and the PGW 113 in the home network 100b. Note that the visited network 100a takes the initial contact with the home network 100b, and not vice versa. The dotted arrows in FIGS. 2a and 2b illustrates optional steps. The method illustrated in FIGS. 2a and 2b comprises at least some of the following steps, which steps may be performed in any suitable order than described below:
Step 201
This step is seen in FIG. 2a. The UE 101 initiates the Attach procedure by the transmission, to the eNB 105, of an Attach Request message.
Step 202
This step is seen in FIG. 2a. The eNB 105 derives or selects a new MME 108 and forwards the Attach Request message to the new MME 108.
Step 203
This step is seen in FIG. 2a. The new MME 108 derives the old MME/SGSN 108 address and sends an Identification Request message to this old MME/SGSN 108.
Step 204
This step is seen in FIG. 2a. The old MME/SGSN 108 verifies the Attach Request message and then responds with an Identification Response message to the new MME 108.
Step 205
This step is seen in FIG. 2a. If the UE 101 is unknown in both the old MME/SGSN 108 and new MME 108, the new MME 108 may send an Identity Request message to the UE 101 to request the International Mobile Subscriber Identity (IMSI).
Step 206
This step is seen in FIG. 2a. The UE 101 may respond with an Identity Response message to the new MME 108 with the IMSI.
Step 207
This step is seen in FIG. 2a. If no UE context for the UE 101 exists anywhere in the network, if the Attach Request (sent in step 201) was not integrity protected, or if the check of the integrity failed, then authentication and Non-Access Stratum (NAS) security setup to activate integrity protection and NAS ciphering may be executed. This step involves the UE 101, the new MME 108 and the HSS 123.
Step 208
This step is seen in FIG. 2a. The new MME 108 may send an identity request message to the UE 101, and the UE 101 sends an identity response message to the new MME 108. The identity response message comprises the identity (e.g. IMSI) of the UE 101.
Step 209
This step is seen in FIG. 2a. The new MME 108 may send the ME Identity Check Request message to an Equipment Identity Register (EIR) 130. The EIR 130 may respond to the new MME 108 with an ME Identity Check Acknowledgement (Result) message.
Step 210
This step is seen in FIG. 2a. The new MME 108 may send a Ciphered Options Request message to the UE 101 to retrieve the Ciphered Options i.e. Protocol Configuration Options (PCO) or Access Point Name (APN) or both, from the UE 101.
Step 211
This step is seen in FIG. 2a. The UE 101 may send a Ciphered Options Response message to the new MME 108 with the Ciphered Options.
Step 212
This step is seen in FIG. 2a. If there are active bearer contexts in the new MME 108 for this particular UE 101, the new MME 108 may delete these bearer contexts by sending a Delete Session Request messages to the Gateways (GW) involved, e.g. the SGW 110 and the PGW 113.
Step 213
This step is seen in FIG. 2a. If a PCRF is deployed, the PGW 113 may employ an IP-Connectivity Access Network (IP-CAN) Session Termination procedure to indicate that resources have been released. The PCEF initiated IP-CAN session termination procedure involves the PGW 113 and the HSS 123.
Step 214
This step is seen in FIG. 2a. The GWs (e.g. the PGW 113 and the SGW 110) may acknowledge the Delete Session Request message in step 212 by sending a Delete Session Response (Cause) message to the new MME 108.
Step 215
This step is seen in FIG. 2a. The new MME 108 may send an Update Location Request message to the HSS 123.
Step 216
This step is seen in FIG. 2a. The HSS 123 may send a Cancel Location message to the old MME/SGSN 108.
Step 217
This step is seen in FIG. 2a. The old MME/SGSN 108 may acknowledge the Cancel Location message by sending a Cancel Location Ack to the HSS 123.
Step 218
This step is seen in FIG. 2a. If there are active bearer contexts in the old MME/SGSN 108 for this particular UE 101, the old MME/SGSN 108 may delete these bearer contexts by sending Delete Session Request messages to the GWs involved (e.g. the PGW 113 and the SGW 110).
Step 219
This step is seen in FIG. 2a. If a PCRF is deployed, the PGW 113 may employ a PCEF initiated IP-CAN Session Termination procedure to indicate that resources have been released.
Step 220
This step is seen in FIG. 2a. The GWs (e.g. the PGW 113 and the SGW 110) may return a Delete Session Response message to the old MME/SGSN 108.
Step 221
This step is seen in FIG. 2a. The HSS may acknowledge the Update Location message by sending an Update Location Ack message to the new MME 108.
Step 222
This step is seen in FIG. 2a. The new MME 108 may derive or select a SGW 110 and then it may send a Create Session Request message to the selected SGW 110.
Step 223
This step is seen in FIG. 2b. The SGW 1105 creates a new entry in its Evolved Packet System (EPS) Bearer table and sends a Create Session Request message to the PGW 113.
Step 224
This step is seen in FIG. 2b. If dynamic Policy and Charging Control (PCC) is deployed and the Handover Indication is not present, the PGW 113 may perform a PCEF initiated IP-CAN Session Establishment procedure. The IP-CAN Session Establishment procedure may involve the PGW 113 and the PCRF 125.
Step 225
This step is seen in FIG. 2b. The PGW 113 returns a Create Session Response message to the SGW 110.
Step 226
This step is seen in FIG. 2b. If there is no handover, the PGW 113 may send first downlink data to the SOW 110.
Step 227
This step is seen in FIG. 2b. The SGW sends a Create Session Response message to the new MME 108.
Step 228
This step is seen in FIG. 2b. The new MME 108 sends an Initial Context Setup Request message and an Attach Accept message to the eNB 105.
Step 229
This step is seen in FIG. 2b. The eNB 105 sends an Radio Resource Control (RRC) Connection Reconfiguration message to the UE 101.
Step 230
This step is seen in FIG. 2b. The UE 101 sends the RRC Connection Reconfiguration Complete message to the eNB 105.
Step 231
This step is seen in FIG. 2b. The eNB 105 sends the Initial Context Response message to the new MME 108.
Step 232
This step is seen in FIG. 2b. The UE 101 sends a Direct Transfer message to the eNB 105.
Step 233
This step is seen in FIG. 2b. The eNB 105 forwards the Attach Complete message to the new MME 108.
Step 234
This step is seen in FIG. 2b. After the Attach Accept message and once the UE 101 has obtained a PDN address, the UE 101 may then send uplink packets towards the eNB 105 which may then be tunneled to the SGW 110 and the PGW 113. Uplink may be described as the direction from the UE 101 to the eNB 105, and downlink may be described as the direction from the eNB 105 to the UE 101.
Step 235
This step is seen in FIG. 2b. The new MME 108 sends a Modify Bearer Request message to the SGW 110.
Step 236
This step is seen in FIG. 2b. If the Handover Indication is included in step 235, the SGW 110 may send a Modify Bearer Request message to the PGW 113 to prompt the PGW 113 to tunnel packets from non 3GPP IP access to 3GPP access system and immediately start routing packets to the SGW 110 for the default and any dedicated EPS bearers established.
Step 237
This step is seen in FIG. 2b. The PGW 113 may acknowledge by sending a Modify Bearer Response to the SGW 110.
Step 238
This step is seen in FIG. 2b. The SGW 110 acknowledges by sending a Modify Bearer Response message to the new MME 108.
Step 239
This step is seen in FIG. 2b. The SGW 110 may send its buffered downlink packets to the UE 101.
Step 240
This step is seen in FIG. 2b. The new MME 108 may send a Notify Request message to the HSS 123.
Step 241
This step is seen in FIG. 2b. The HSS 123 may send a Notify Response message to the new MME 108.